The present invention relates generally to sensors and, more particularly, to electromagnetic sensors used to provide location data in medical procedures.
Electromagnetic sensors may sometimes be used in medical procedures. For example, electromagnetic sensors may be incorporated into catheters or other invasive medical devices used, e.g., for electrophysiological applications, to provide very accurate location data. An example of a system that employs an arrangement of orthogonally positioned sensors for providing three-dimendional position coordinates of the location of the sensors is shown and described in PCT publication WO 00/10456, entitled xe2x80x9cIntrabody Navigation System for Medical Applications,xe2x80x9d which is hereby expressly and fully incorporated herein by reference.
In particular, an orthogonal arrangement of at least three sensors, which are typically composed of a metallic core, such as copper coil, is mounted in close proximity to each other on a locatable device, such as at the distal end of a catheter, and is used to receive (or transmit) electromagnetic wave energy in a classic x-y-z Cartesian coordinate system. In this manner, the moving object can be located with respect to a fixed frame of reference, thereby allowing an attending physician to locate the moving object within the body of a patient. During assembly of the sensor arrangement, the respective locations of the orthogonal sensor devices are placed in an optimum orientation, and are then fixed, typically by encapsulating the entire sensor arrangement in a potting or adhesive material, such as an ultraviolet (UV) adhesive, resulting in a self-contained and easily packagable and transportable sensor device.
Despite this encapsulation process, these sensor devices may experience a loss of accuracy as a result of changes in environmental conditions. Specifically, it has been noted that various temperature cycling (e.g., when the sensor is exposed to extremely high and/or low temperature during shipping, storage, use, and assembly) appears to reduce the accuracy of the sensors. Thus, the sensors often need to be recalibrated when the sensors are exposed to temperature cycling, and often must be discarded if found to be inaccurate. In the worst case, the object to which the sensors are mounted must also be discarded if the sensor assembly experiences a loss of accuracy after it is installed on the object. Therefore, it would be highly desirable to have an electromagnetic sensor that is able to retain its accuracy during temperature cycling.
The present inventions are directed to sensor assemblies that are capable of retaining their accuracy during temperature cycling.
In accordance with a first aspect of the present inventions, a sensor assembly comprises a number of sensor elements that are covered by an encapsulant exhibiting a coefficient of thermal expansion approximately equal to that of the sensor elements. In this manner, the sensor elements and encapsulant expand and contract at substantially the same rates when exposed to environmental fluctuations, thereby minimizing displacement of the sensor elements. In order to ensure approximate equality between the first and second coefficients of thermal expansion, the curable adhesive can be conveniently doped with an additive. For example, if the coefficient of thermal expansion of the curable adhesive is greater than the coefficient of thermal expansion of the sensor elements, an additive exhibiting a coefficient of thermal expansion less than that of the sensor elements can be used. By way of non-limiting example, ceramic, e.g., aluminum oxide, magnesium oxide, or silicon oxide, can be used for this purpose. Alternatively, the adhesive may inherently exhibit a coefficient of thermal expansion that is approximately equal to the coefficient of thermal expansion of the sensor elements, thereby obviating the need to dope the adhesive.
In accordance with a second aspect of the present inventions, a sensor assembly comprises a number of sensor elements. The number of sensor elements is covered by an encapsulant comprising a curable adhesive and an additive. The additive exhibits a coefficient of thermal expansion less than that of the sensor elements. For example, ceramic material, such as aluminum oxide, magnesium oxide, or silicon oxide, can be used. Thus, if the coefficient of thermal expansion of the curable adhesive is greater than that of the sensor elements, the coefficient of thermal expansion of the encapsulant can be lowered to more closely match that of the sensor elements.
In accordance with a third aspect of the present inventions, a medical sensor assembly comprises a number of sensor elements that are configured to provide location data during medical procedures. The number of sensor elements is covered by an encapsulant comprising a curable adhesive and an additive. The additive is used to modify the coefficient of thermal expansion of the encapsulant to a suitable value. The coefficient of thermal expansion of the additive can be less than that of the sensor elements. In this case, ceramic material, such as aluminum oxide, magnesium oxide, or silicon oxide, can be used as the additive. The coefficient of thermal expansion of the additive can alternatively be between that of the adhesive and sensor elements. In this case, glass microspheres can be used as the additive. Thus, in either case, if the coefficient of thermal expansion of the curable adhesive is greater than that of the sensor elements, the coefficient of thermal expansion of the encapsulant can be lowered to more closely match that of the sensor elements.
In accordance with a fourth aspect of the present inventions, a method of making a sensor assembly comprises selecting a number of sensor elements and selecting an encapsulant having a coefficient of thermal expansion that is based on that of the sensor elements. By way of non-limiting example, the coefficients of thermal expansion of the sensor elements and encapsulant can be approximately equal. The coefficient of thermal expansion of the encapsulant can be modified to a suitable level by doping an adhesive with an additive. The coefficient of thermal expansion of the additive can be less than that of the sensor elements. In this case, ceramic material, such as aluminum oxide, magnesium oxide, or silicon oxide, can be used as the additive. The coefficient of thermal expansion of the additive can alternatively be between that of the adhesive and sensor elements. In this case, glass microspheres can be used as the additive. Thus, in either case, if the coefficient of thermal expansion of the curable adhesive is greater than that of the sensor elements, the coefficient of thermal expansion of the encapsulant can be lowered to more closely match that of the sensor elements. The encapsulant is then ultimately used to cover the number of sensor elements.
In accordance with the aforementioned aspects of the invention, the sensor elements can comprise a metallic material, such as copper coils. The number of sensor elements can be one or more. If the number is greater than one, the encapsulant can cover the sensor elements to form an integral assembly. In medical applications, the sensor assembly is preferably configured for installation on a catheter or other device that can be introduced into the vasculature of a patient, and operated to provide location data during medical procedures.